USC Speech Codec

[pchapman]

Hi,

I'm currently trying to decode some G.711 u-law 8-bit audio using the USC G729 codec. The ultimate goal is to package the encoded bit stream into a payload format that can be transmitted over RTP.

To test the functionality of the USC I'm using the example code from the USC API. The result of the encode gives 80-bits (10bytes) bitstream.

I then pass the output buffer to the decode function. When I try to play out the encoded/decoded data over RTP, the payload is not correct.

So I have a few questions....

1. What format is the decompressed data in? 16-bits representing a byte?

2. What would have to be done to the bitstream, in order for it to be a valid G.729 RTP packet payload?

Any help would be greatly appreciated.

Kind Regards,

Paul Chapman

Junior Software Engineer

[Igor_B_Intel1]

Hi, Paul.

USC G729 codec doesn't support 8 bit u-law input. For such kind of bitstream please use USC G711 codec.

As for USC G729 codec:

>1. What format is the decompressed data in? 16-bits representing a byte?

16 bits signed word (two bytes).

>2. What would have to be done to the bitstream, in order for it to be a valid G.729 RTP packet payload?

Nothing special. USC G729 codec operates with 10ms frame. G729 RTP profile needs 20 ms data. So in case of VAD turned off you need to concatenate two frames.

Igor S. Belyakov

[pchapman]

Thank you for your reply. This will explain the problems I have been having. What input is valid for the G.729 codec? Could I call ippsMuLawToLin_8u16s()to change the input from u-law to liniar input?

Regards,

Paul Chapman

Message Edited by pchapman@telesoft-technologies.com on 04-25-200612:31 AM

[Igor_B_Intel1]

Yes, of course you can use ippsMuLawToLin_8u16s()function toconvert from u-law to liniar PCM.

On the other hand USC G711 codec can do this convertion and additionally it has got VAD and PLC modes.

Igor S. Belyakov